Method and apparatus for indicating frequency coincidence between alternating currents or voltages



D. B. SMITH METHOD AND APPARATUS FOR INDICATING FREQUENCY Nov. 2, 1948.

coINcIDENoE BETWEEN ALTERNATING CURRENTS OR VOLTAGES 7 Shets-Sheet 1 Filed Deq. 50, .1944

Nov.' 2, 1.948.

D. B. sMrrHV 2,452,960 METHOD AND APPARATUS FOR INDIGATING FREQUENCY COINCIDENGE BETWEEN ALTERNATING GURRENTS OR VOLTAGES Filed Dec. 50, 1944 '7 Sheets-Sheet 2 INVENTOR.

METHOD AND APPARAT'US FOR INDICATING FREQUENCY COINGIDENCE BETWEEN ALTERNATING CURRENTS OR VOLTAGES Nv.2,194s. D B SMWH 2,452,960

'7 Shee'ts-Sheet 5 Filed Dec. so. 1944 IN V EN TOR.

Nov. 2, 1948. D, B, SMITH 2,452,960

METHOD'AND APPARATUS Foa INDICATING FREQUENCY GOINCIDENCE BETWEEN ALTERNATING CUERENTS 0R voL'rAGEs 7 Sheets-Sheet 4 Filed Dec. 30, 1944 C/m/v/VEL 2 FREQUENCY D. B. SMITH 2,452,960 METHOD AND APPARATUS FOR INDICATING FREQUENCY COINCIDENCE BETWEEN ALTERNATING CURRENTS OR VOLTAGES Nov. 2, 1948.

Filed Dec. 50. 1944 Nov. 2, 1948. D. B. sMlTH 2,452,960 METHOD AND APPARATUS FOR INDCATING FREQUENCY COINCIDENCE BETWEEN ALTERNATING CURRENTS OR VQLTAGES Filed Deo. 3o, 1944 v sheets-sheet e -F/G; 2. F7C-.5.

)f1/G. 4. H65;

- VOLT/46E l. l ZERO 'F/efQz/fA/cy INVENTOR. DHV/D B. 5M/7H NOV. 2, 1948. ,SMITH 2,452,960 METHOD'AND APPARATUS FOR INDICATING FREQUENCY GOINGIDENGE BETWEEN ALTERNATING CURRENTS 0R VoLTAGEs Filed Deo. 3o, 1944 7 sheets-sheet 7 ZERO middle-NCL 'INVENTE i intervals Patented Nov. 2, 1948 2,452,96t l f METHOD AND APPARATUS FOR INDICATING FREQUENCY COINCIDENCE BETWEEN AL-` TERNATING CURBENTS OR VOLTAGES David B. Smith, Flourtown, Pa., assigner, by mesne assignments, to -Philco Corporation,

Philadelphia, Vania Y Application December 30 24 Claims. (Ci. 2250-39) `This `invention relates. primarily to asystem and method for accurately and usefully indicat-` ing frequency coincidence (zero beat) as respects two high frequency electricwaves--more especially where random' phase relations `obtain be` tween the two waves, and where the frequency of at least one of said Waves `is continuously changing; and it also relates to a system and method of -calibrating high frequency instruments such as frequency meters of the-signal generator type, andother devices of precision character.

One `of the principal objects is to reduce the time required to calibrate certain high frequency precision instruments.

Another object is to devise means operative 1to produce electric pulses in` response to and concurrently With zero beats` of substantially` infinitesimal durationwhich pulses are consistently suitable for triggering a recording mechanism, irrespective of the phase relation which may happen to obtain between the interacting signal waves when their frequencies instantaneously coincide.

`A further object is to provide an automatic calibrator for high frequen-cy meters and the like,

Vwhich is capable of operating rapidly while at the same time ensuring a very high order of precision.

Still another object is to provide a high frequency Calibrating instrument which is stable in operation and not difficult to maintain.

An additional object is'to provide improved means for detecting occurrences of zerobeatbetween coacting Waves. i

The specific disclosure herein pertains to an automatic calibrator Whose function is to record accurately and speedily a large number of dial readings of a frequency meter or signal generator-each of which dial readings corresponds to an individual frequency. The frequency meter has twowave bands, namely: 125 k. c. to 250 k. c., andl 2.0 megacycles to 4.0 megacycles, respectively,

Pa.,` a `corporation of Pennsyl- 1944, serial Nq. 570,714

computed and recorded after the `dial readings corresponding to the 327 principal calibration `points have been` determined precisely by the automatic calibrator. l l

The standardsignal generator with which comparison ismade is designed to `generate successively two distinct bands of harmonic frequencies,

each band `extending `from 2.01megacycles to4.0

megacycles and composed of harmonicsoccurring at intervals oflO` kilocycles and 16 kilocycles re` spectively. That is to say, one band `comprises a series of discrete harmonic frequencies of` 2000` k. c., 2010 c., 2020 kpc., etc.,;up to 400W-kof#- making" a totalof `201---while the otherband comprises harmonic frequencies of 2000 kl c.,20l6 k. c., 20321k; c., etc., upto 4000 k. c making a total of 126-thevgrand total` being327, which equalsthe number of principalcalibratiolns per instrument. l i l y In the operation of the; system the tuning shaft of the instrument `under calibration is `power driven at a uniformspeed from one end of its dial scale tolthe other, and said instrument (which is,` itself asignal generator) puts out a signalwave which varies in frequencycontinuou-sly from 250 k. c. to 125 k. c.; and thereafter,

and it is a prescribed requirement that the low A'bandV 125-250 k. c.)-be calibrated at 1 k; c. intervals bycomparison with signals from a `standard, signal generator, and that` the high band be similarly calibrated at l0 koe. intervals. :Accu- 1 l Vracy within .01 percent was specified with respect to each of the aforementioned calibration points-there being 327 such calibrationpoints per instrument. ln addition, intermediate points at 0.1 k. c. intervalsfor-the low band and l k; c.

` for the high band are calibrated by interpolation. That is to say, the dial readings corresponding to intermediate frequenciesare '-6' upon reversal of" rotation, it puts' outA a signal which varies in `frequency continuously from 2.0-4.0 megacycles-Lthe first mentioned band of frequencies (the low band) being multiplied sixteen times in orderto` produce a band o f 2.0i to 4.0megacycles for` comparisonwith the corresponding standard signal generator band.` Itis important to observe that the high band` is `the sixteenth harmonic of the low band. l

The high frequency outputlof the instrument under calibration combines, first, with each consecutive harmonic of one band and then with eachconsecutive Iharmonic of the other ,-band of the `two standard signal generator bands'previouslymentioned7 and there is produced a momentary or instantaneous zero beat each time the output frequency ofthe instrument under calibration `coincides with one of said harmonica@ The occurrence of a z'erobeat means,` obviously,

- thatthe instrument under calibration is genera't` ing l a `signal wave of i exactly the .same -frequency or a sub-'multiple thereof) las one of 'the-.-harmonies of thev standardsignalggenerator ,and

shaft `of the instrunlenl',` undergoing` calibration isbeing driven continuously-which renders itA `impracticable to`obtain i reliable 1 indications zero beats occur, it may be that no serious difculty would be encountered in obtaining usable indications of the momentary Zero beats, which could be translated automatically `into a -per-manent record ofthe dial settings corresponding to each of the 327 principal calibration frequencies. But, unfortunately, the phase'relation between the two beating wavesis entirely random. That is to say, the phase angle may beA anything from zero to 360. For that reason it is not practicable to obtain directly from the beat wave -a .voltage or current pulse which will in every case be of such character that it can be distinguished from non-Zero-beat pulses. Y tion affords an effective solution of that problem iin that it provides an indirect means ywhereby instantaneous zero-beat occurrences are caused toproduce substantially uniform distinguishable pulsesirrespective of the phase angle which may obtain between the two interacting Waves when their frequencies coincide.

The solution `of the :problem may be resolved into a series rof major steps, the first of which consists in deriving two beat waves which are Ypreferably identical ybut 'dilfer in phase by 90 deygrees---the two quadrature-related beat waves `each being generated by combining wave energy from the frequency meter or other instrument under calibration with wave energy lfrom the standard signal generator. At each frequency coincidence the two beat waves are momentarily of zero frequency, but their amplitudes differ by 90 degrees, due to the aforesaid phase diierence.

The second step consists in so operating, in-

dividually, upon each of the -two quadraturerelated beat waves that a sharp voltage change is brought about in response to the low rate of change of frequency which occurs at andim- Imediately adjacent zero frequency. This step involves, preferably, integration or partial intergration of the two beat wavesjbut it can be accomplished, albeit less simply, by differentiation or partial differentiation or by other equivalent `processes capable of translating frequency changes into amplitude changes.

The third step consists in vimpressing simultaneously upon separate square-law detector ,stages the quadrature-related integrated voltages produced by the aforesaid second step and by yadding theoutputs of the two square-law detector stages. The result o-f this operation is, in each instance, a voltage `pulse of' predetermined amplitude, wave-front and polarity in vstep with each successive zero 'beat and capable of being utilized to eifect an. accurate 'permanent record ofthe 'frequency meter dial setting for each frequency at-which automatic calibration is required.

Another feature of this invention is predicated upon my discovery that increased accuracy of calibration 4can 'be achieved by frequency-multiplying the outputs of the frequency meter and of the standard'signal generator and, further,

'that 'said outputs can be vheterodyned down to a convenient intermediate frequency Aprior to `fre- The present invensystem; Y

quency multiplication without introducing any inaccuracy; and that by virtue of such heterodyne it is possible to maintain the aforementioned quadrature phase relation throughout the Calibrating cycle without any rea-djustment being necessary, and with aconsiderable gain as respects simplification of tuning.

Other features and objects of this invention will be .noted as the detailed description pro gresses;fand ,it will become apparent that certain `'novel features and .subcombinations are prospec- Aand Joseph Tellier and possibly other inventors who collaborated inthe development of the automatic calibrator described Ain the "Electron-ics article. v

The present application isa continuationin part of the cao-pending joint application'of Joseph Tellier and the :present applicant, David B. Smith, Serial No.53s,07o, filed May 30, V1944.

As regards the subject matter being claimed in the present application, `the present applicant is the sole inventor' of such subject mattenthe said Joseph Tellier having been'included asf'a co-inventor thereof .in the prior application through inadvertence or'niistake.` This is coniirmed by the said Joseph Tellier in a statement attached l-to and made a part of the rpresent application. i

Referring to the drawingsz y Fig. l is a block diagram'covering the en ire Fig. 2, 3, 4 and 5, jointly, constitute a circuit diagram of what will be called the electricalend of the system; 'Y

Fig. 6\ is a block diagram illustrating how Figs. 2, 3, 4 and5, should be arranged to forma cornplete and connected circuit diagram;

Fig. '7 is a composite graph of several bea waves, showing the effects of various phase angles upon the instantaneous amplitude at zero frequency;

Fig. 8 is a composite graph illustrating the results of integrating each ofthe b eatwaves of `As previously indicated, this frequency meter covers two bands, Vnamely -25D k. c. (the f low band)A and 2.04.0 rnegacycles l(the high l'Vo band),l The designationfrequency meter -is employed here in order vto conform to ILS. Army terminology; but the instrument lreferred to 'is what'V would more commonly be called a signal generator. i

Calibration is accomplished by starting at the 25() k. c. end of the low band and turning the v dial ofthe' meter ata constant speed untilit reaches the 125 k. c. end, andY then switching the meter tonthe high band and.reversi-ng the vdirection of '-rotationf-'fThe dial iis-,then'tured together, may constitute a single generator which :isadjusta'ble to propagate the two separate seri-es of harmonics consecutively; but to `simplify the lillustration and description I have chosen to show two separate and distinct' generators.

A two-.wayiswitoh il, which may be automatically opera-ted, serves to shift from onestande .ardksignal `generator to the other.- When in use,

the flow band` standard signalfgenerator ,puts out,

wcontinuously, a series of `harmonic frequencies `which are multiples of sixteenwlilocycles.@The lowest harmonicr employed, in this band' isthe 125th-which has a frequency of 2060 k. cpor` 2.0

.megacycles vTheneXt employed harmonic in the iowzband is the lZBth-which vhas a frequency of 201.6 k. c.; and the next, the `127th, has a `frequency `16 k..c. higher or 2032` k. c. Thus, the

lowband consists 4of 126 harmonic frequencies spaced `at 16 k. c. intervals', extending `from 2000 k. c. zto 4000 k. c. It is to be understoodV thatv although the low band of the frequency meter is `12B-250 k.. c., the output of the low band standard signal generator is 2.0-@ rnegacycles which is the 16th multiple :of 1125-256' it. c. It will now be` observed that the 16 k. c. intervals between successive `low band harmonics put out by standard lsignal generator 2 are proportionate' l* tol wk. c. intervals in the frequency meterl'ow band.

i .Now .let us consider the high band standard sig# p nal: generator 31. When this is in operation itputs` out, continuously, a series ofharmonie'frequencies which are multiples of ten ki'locycles. Theirl'owest `employed harmonic of this series i's'th'e twohundredth, whichy has a frequency of '2:0003 k. onor 2.0 megacycles. The next employed .harmoniczin -the high band is the 20lst, which has `a frequency i of 20:10 k. c. or 2.01 megacycles. Thus, .in passing through the high band there is a `harmonic at eachsucceeding ten kilocycle `interval instead of each sixteen kilocycle interval. i

Calibration of the frequency meter is -accomplished by producing an indication each time the output frequency of said meter is either equal to one ofthe signal generator high hand harmonics (as in calibrating the high band of the zmeteri),

or is equal to` onefsitrteenth .fof one lof thesignal generator low band` harmonics; 'and' this is ldone by heterodyning the frequency meter Aoutput with the outputs of the ystandardssignal generators consecutively to produce a Zero beat` for each' successive frequency coincidence. Thus, in Calibrating .the low band zero beats are produced `at 1 c.

intervals betweenlz k. c. and 250k, c.. In order todo `this itis necessary tormultiplyl they lowv band output frequency or the frequency meter :byA `sixteer1-thus producing a 2.04;0m`egacycleiband.

In Calibrating the highband', zero beats are produced at `10. leso. intervals between. 2.10' mega cycles and 4 0 megacycle's.

At the outsetthe frequency meter isl adjusted `manually to generate 250 k. c. atta predetermined dial reading; and is further adjusted in the 'high band to generate 4.0 megacycles at the-same dial reading.

Atkevery `zero beat there is produced at lilioii'ui J,.5\.-'6F.ig...3,) gvoltage 'pulse capable"v ofbeing ent!y '75 phase of either signal.

reach frequencyl ployed to 'record 'the .frequency meter dial reading thereof. 1 i r l i I The usual practice is to make a log book for eter, in which isrecorded the dial readings corresponding to the several Zero beat frequencies, as determined 'by the automatic calibrator; and, in addition, further dial readings are computed at one-tenth intervals betweenthe readings corresponding to `zero'lbieat'frequencieswhich latter" readings are also entered inthe' log book'. Thergraph of dial readings'plotted against frequencies is sumciently `linear between consce- 'utive zero beat frequencies to enable the intermediate readings to be interpolatedk without eX- cessive error.

From what has been said vit should be apparent t that each zero beat or lfrequency coincidence is of infinitesimal duration; and inasmuch as the shaft of the frequencymeter is moving continuously at a substantial` speed itl is evident that each "suc-l cessive period during which the beat frequency is close to Zero is of exceedingly short ,"durat'irin.

This appears to render it impossible ltnrvanypre` viously known method of which I am awaretcobtain sufficiently accurate indications of the Zero beat points; and this is doubly true because there is no fixed phase relation between the `component waves at and adjacent Zeronbeati To illustrate this point, reference is madeto Fig, '7, which com.u prises a number of superimposed `sine-wave graphs of beat waves which maybe assumed to have resulted from combining` and detecting pairs of component waves-which pairs yare identical except as to the phase relation `which happened to obftain `between them at frequency coincidence.

,Curve A represents a beat wave producedby components which were exactly in phase at frequency coincidence. At Zero frequency the amplituderof curve A is maximum. Curve B represents a beat 4wave produced by the same components as curve A but having a phase differenceof Herewthe amplitudeat zero frequency is something less than maximum. `Curve C isindicative of what happens at frequency coincidence (Zero beat) when the phase angle'between the component waves, at that instant, is It will be :noted that the amplitude in the latter case `is zero. Since `there are an infinite number of possible phase angles, there are, correspondingly, an infinite number vof possible beat wave forms-those shown in Fig. 7 being merely representative.

`In addition to curves A, B.; and1 C, there are;

also included 'in Fig. '7 three incomplete curves marked D, E, and F, respectively. Each of these latter curves, if completed, would be the converse, respectively, of curves A,.B, and C; and they result from phase angles of 135 and -90. `.It is clear fromlFig.` 7` that the beat wave voltage at frequency coincidence may be either positive,

. negative or zero and that the probabilities. fas` between positive and negative, are` equaL.` since the phase :anglaiswhollyfortuiteus 1 i .i

The equation of each of the curve of Fig; V'7 is:

Where ezb--instantaneous value of zero beat voltage. Ezb=peak value of ezt.

R=rate of change. `of frequency of frequency meter'under calibration. o=difference in phase at instant of zerobeat'between the two component signals forming the beat wave--without regard to the absolute t=time measured from the instant of zero beat.

The following additional notation is employed in the ensuing derivation of Equation 1.

ver=instantaneou's voltage output of frequency `meter under calibration. IEr=peak value of ef.

es=instantaneous Voltage output of standardsignal.

lllls=peak value of es. l

` 1Ds=periodicity of (2n-f) of standard signal (a constant). R=rate of change of dwfdwf n wher-ae p i f=phase angle of frequency meter signal at instant of zero beat (equals ef at instant of zero beat). &=phase angle of standard signal (equals es at instant of zero beat) y Since d wir it follows that 0f=fwfdt Then: ef-:Ef Sin 0f=Ef Sin fwfdt wf: ws-I-Rt fafdt= f w,+Rt)da=w,+%z4-f 8/:Ef SII (wsl'f-gz-I-qf) l Similarly,

e,=E, sin (unt-F42.) (3) Now, let

G=wf+t2+f (4) ifrhen,

(Ef sin a) (Ef sin b) =E/E, sin a sin b (6) :mE/2E# [00s (frb) 00S a+b)] 7) which follows from the trigonometric identity:

(a-I-b) l, K1 being In Equation` (10),-l the expression (Ef sin a) YES sin b) is equivalent to eab of Equation (l),

the expression defines the amplitude of the beat wave and is l equivalent to Ezb in Equation 1) the term denes a low frequency component, and the term cos (zwzrwfw.)

denes a high frequency component which is ltered out and can, for that reason, be disregarded.

It should now be clear that the major problem arose from the fact that it is necessary, in order to save time, to drive the tuning shaft of the frequency meter continuously, and the further fact that the phase vangle at zero beat between the component high frequency waves vis ventirely random, wherefore the beat waves cannot be directly utilized to produce pulses reliably coinciding with the successive Zero beat occurrences. The solution of this major problem can be explained most advantageously as an incident to the description of that portion of the calibrator` systemillustrated diagrammatically in Figs.'2-5.

The system of Figs. 2-5 comprises two channels,r identified aschannel #l and channel #2, respectively. Channel #1 embraces, in addition'to the two standard signal generators," a radio frequency amplier 6, a first-mixer 1, an intermediate frequency amplifer 8, a frequency multiplier 9 and an amplitude limiter I0. f Channel #2 comprises, in addition toithe frequency meter under calibration, a dual-purpose stage II, a rst-mixer I2, an intermediate fre'- `quency amplifier I3, a frequency multiplier I4,

and an amplitude limiter I5. A local oscillator I6 is common to both channels. A second-mixer I'I and another second-mixer I 8 are each -fed from both channels, jointly, and, therefore, cannot be said to be individual to either channel. The output of second-mixer I'I is passed through an integrator I9 and thence to the grid of a phase inverter tube 2D which controls a dual-tubey square law detector stage 2I.

The output of second-mixer I8 is likewise passed through an integrator 22 and thence to the grid of a phase inverter tube 23 which controls a dual-tube square law detector stage 24.

The outputs of. both square law detector stages 2| and 24 are impressed upon opposite terminals of a potentiometer 25 which functions as a voltage adder; and, as a result of adding the outputs-of the two square law detector stages, there appear at point 5, contemporaneously with the successive zero beats, recurrent negative pulses of uniform amplitude, which are capable of being employed to effect actuation of a mechanism for recording the coincident dial readings.

The recording mechanism, per se, may take any one of several optional forms and does not constitute a part of the present invention, except as an element of an entire system.

Since the pulses appearing at point 5'are relatively weak, and Vit is necessary to obtain considerably stronger pulses for triggering the prehigh band ferred recorder, there is provided a blocking oscillator 26 which is capable of producing a powerfulgpulse in response to a weak triggering pulse. The" blocking oscillator is'preceded by a 'phase inverter 21 which is necessitated by the fact that the blocking oscillator requires positive triggering pulses, whereas the pulses at point 5 are negative. g

The dual-purpose stage II operates, without switching, to pass unaltered the 2.0-4.0 megacycle output of frequency meter I, 'and to frequency multiply by sixteen the -250 k. c. lowL-.band output of the frequency meter. Hence, thekoutput of the -dual-purpose stage is a 2.0-4.0

encased.:

inegaoycle` band, `irrespectiverofi'` whether the: in.- put"` -theretoi isi the4 frequency.` meter" low. band: or` the frequency meter higlrband;` The output'` signalsloffthe duals-purpose` stage producedy by the 1Nvo`bands:` are of" thesame order of. magnitude'.`

rlihe first tube; 215i, provides considerablelgain: on theklow band, but attenuates.thexlfiighl band` due to' bypass capacitors` 2%', 295 shuntingV theV plate loadl Tube 3'@ functionsasan amplifier on the` high' band; thusvoifsetting the high bandy loss in.`

thelrstwstage v l lSince-transfornfrer 3l'lis tuned tothe hightbandf,

it functions, in conunctionlwithtube 3b, as a the plate of tube 3 3 which tube forms a part" of'local' oscilla-tor It. Theyoscillator shown isof theA `I-Iartln'ey',type and the plate or tulbe-` 3?` is electron-coupled thereto;

' The outputof the dual-purpose stage` is' connected through conductor 34:l to. the signal; input gridcfrstlmixerl via a tuned gridA circuit 35; and the oscillator injection grid.' of the same mixer is connectedtolocal'.oscillator".f6 via conductorl 36.

Thelocal oscillator is gang-connected .to the frequency meterand tothe radio frequency amplifier 5; as Well'as the' tuned input circuits of'both, first-inixersf-all of which is indicated byV the broken intercoupling lines 311". f By virtue of" this ganged` connection the frequency of the signal at the output of dual-purpose stage l I is always the same as the frequency to which amplifier 6 is tuned` Sand l-ilrewise the: frequency: to. which the inputs of thegtwo7 first-mixers are` tuned. But

local oscillator i6 is so adjusted thatat all times its output is of higher frequency than that ofthe other signalsbei'ng. impressed upon-.the two rst mixers-and this by an amount equal to the de sired intermediate frequency. It. was found that 480 k; c. was a desirable intermediate frequency. Hence, the local oscillator is designed to cover a beats, which would be likely to cause untimelyK operation of the recorder, it is necessary to restrict the band width of I. F. amplifier 8 tol ap.- proximately i. 5k. c.

In both channels the intermediate frequencyampliflers are followedby frequency multipliers. These are identied' by reference numerals 9 and i4, respectively, and they are, specifically, frequency triplcree-although` any` desired. muitiplicai tion could be. employed. `An explanation as to the reason for' providing the frequency multipliers can best be introduced. atila subsequent point in this specir'i'cation` andzwill accordingly. be deferred The outputofthe frequency tripler Illisvacon-` tinuous` wave of 144m k.. c. while tiheroutputfv ot tripler. 9'; is:-` aconsecutive series of: intermediate;

. frequency signals i each varying.V infrequency, with time, from 14251;; c. to 14551lec-orvice versa@ and having a center frequency; oflzlildn c 'llhe objectiue is to; obtain a; usable indicatin corresponding to eachjperiofdical instant whensthe frequency ot the output of. tripler 91 reaches, momentarily, 174%407 k. c., anduthus.coincides with. the cutputxfrequency: of. channelettZe-whiclr` condition,obtainswheneverand only when the outputl ot thezrneterunder calibration (ora.` 16th multiple, in.` the case; ofrthel low band). islidentical` in] freiquency with one ofthe harmonics from onaon the other stan-dardsignal. generator.`

Due to. the fact thatxthere'isY a very` considerable disparity in` signal strength as between the:

various harmonics propagated; by the. standardl signal generators, and` tolthe furtherrflact thal'nit` is` important that the recurrent` pulses` developed at pointl 52 be ofi'substantiallyuniform magnitude, there. is included' in channeli #il an.A amplitl1de;` limiter t0.` `AnlplincationI inthe LF.. stage B11 or the E; stage Gf, orrboth, iscsuch4 that evemthga Weakest harmonicy wil'lucause then plate` current of? limiter Iixto reachfsaturationt, Thusthepoutput oflimiterz lxllris of'` constant. amplitude irrespective otthestrength of the harmonic;` lncreasedunf: ilomnity.` of the signals.` impressed upon the input; of: limiter la? cankbe. achieved- `through, application; of. automatic volume central voltage tothe gridv ofi amplifier 6s. and. the. signal gridrofmixerf 1:.

v Conductor Baiservesflthis;purpose; Since.itsistdef-` sirable` thattthe outputs: of: thel two: channels bef of.

substantial'ly` equal amplitude,` there is; alsoy in,` leluded-inl channel #i2 an. amplitude limiter'Il'n` Now the: problem which` hasgtopbe; dealt; withzisi@` thatV of beatixccg` togetherl4 andfutilizingthe outputs; of'thetwotamplitude limitersin such away as -.to` obtain all pulse! concurzc'ently:` with` eaclfr` successive frequencyy coincidence-Which.pulse is invariably suiiicientyandl suitable totrigger. arecording mechanism. Thev problem is complicated, inpart by; the.` fact; that eachfrequencyA coincidence or1 f zero` beati isof but instantaneous duraticm and,

i a portion of theA further, by thefactthat-.thettwo combining waves y have; nuiixedi phaserelaticn with' respect to eaeh other at` and adjacent: frequency coincidence` rEhe phase angle between them may be anything.

from zero;- to 36GB. This means that the. ampliL t-ude.A of the beat Wayeat: frequency c oimzidence` will rangeE trom` zero to;maXi'mum---forY which rea-U som it is; incapable oibeingdirectly utilizedto achieve highly accurate; triggering ofja recorder; The; rststepiin, the solution of l the stated prob lem; consists in combining-1 portions: off the outputs` or. the: twoamplttude limiters im such.` away-that; there; are pronti-.ced4 at ancliimmediately adjacent` each` frequency' coincidencey twobeat Waves which.

are identicall except that they are mutually phasef displaccdbtt am. angleoii 9.0.51` This is;accomplishedV by` direetlyy beating; together intsecond-mixer |11'. dutputof limiter Iwith ai purr tien of;` thetoutuut off limiter and@ beating together ins'secondrfmixen t8 another portion of they output of limiter; mand another-portion of the output. `of limiter:` |-5;, whichl latterA is shifted-1 phase l by 90, as compared to that portion of; the. output of limiter t5; which, is impressed `upon second-mixen |11 l One .input grid of mixer ll'l: is fed1di-rectlyfrom the plate of; limiter tsvia conductcrf 39: andsblockina condenser All; whilethe seccndzinputrgrid ofY l i the saine mixer is fed from the secondary of transformer 4|. On the contrary, the two input grids of mixer i8 are fed from the two secondaries of transformers 4| and ft2-one of the last mentioned grids being connected to the secondary of transformer 4| by way of conductor 43 and blocking condenser 44.

Since transformer 42 is loosely coupled and is tuned to 1440 k. c. it follows that the voltage across the secondary thereof is 90 out of phase with the voltage across the primary winding, wherefore the beat frequency output of secondmixer I8 is outof phase, to the same degree, with the beat frequency output of second-mixer l1. And this holds true for all frequencies, including zero.

The output of each second-mixer is a continuous series of vdiscrete beat wave signals, each starting at about k. c. and uniformly decreasing through zero frequency and thereafter uniformly increasing to a nal value of about 15 k. c.

As illustratedby the graphs of Fig. 7 the voltage ofthe .beat waves produced by second-mixers l1 and I8 may ea-ch have any value from zero to maximum when zero beat occurs; but whatever voltage value may happen to obtain at zero beat as respects either one of the two beat waves, the voltage ofthe other will differ from the first by 90. If, for instance, one value happens to be zero the other will be a positive or negative maximum; and

if 'one has an intermediate value the other will have another intermediate value `differing therefrom by 90. One voltage may be positive and the other negative, or both may be positive or both negative. Having produced -two beat waves which are mutually phase-displaced by 90, the next step is to produce from each such wave, separately, a pronounced signal pulse which is coincident with zero beat. That result is here achieved by separately integrating the outputs of the two secondmixers, but it may, alternatively, be accomplished by differentiation of said two outputs, or by any suitable means capable of converting frequency changes into commensurate amplitude changes.

The integrator I 9 which follows second-mixer Vlcomprises a series resistor 45 having a value of`0.33 megohm and a shunt capacitor 46 having a'valu'e'of0.5 mf. Capacitor 46 is, in this particular instance, shunted by a grid-leak resistor 41, which, ofcourse, impairs to some extent the effectiveness ofsaid capacitor, but not seriously. Manif-estly,Y elementsl 45 and -46 cannot afford exact integration, since, in theory at least, that can be realized only with a resistance and capacitance both of whichy are of infinite magnitude. However, there is achieved an approach to complete integraton'which is entirely adequate for the p-urposes of the present invention, and it is to be understood that l:in referring to integrators and integration I do not wish to be construed -as using those terms in the rigorous mathematical sense. Integrator 22 consists of a series resistor 48 and shunt capacitor 49, which latter is shunted by grid-leak resistor l5|). These may be identical with elements 45, 4B and 41 respectively.

The voltage output of each integrator is illustrated graphically -in Fig. 8. Each different phase angle which may happen to obtain between the two component signals forming a beat wave will give rise to' a corresponding integrated voltage curve. Thus, Icurve A', Fig. 8, depicts the result of integrating beat wave A, Fig. 7. Likewise, curve B', Fig. 8, is Ithe graph of the integral of Icurve B etc. Justas there are an infinite number of pos` sible beat waves, Fig. 7, there are similarly an innite number of possible integral curves, Fig. each corresponding to an individual phase difference between the component signals.

It will be observed that some of thecurves of Fig. 8, namely, A,\B, C', rise rather sharply to' the positive side of the zero voltage axis just priory to zero frequency, while -curves D', E', and F drop be1ow the zero voltage axis and are, respectively, the linverse counterparts of curves A', B', and C. If all possible curves were included in Fig. 8, or ai large enough number to be conclusively repre-j sentativa'it would be found that one-half of them would rise above the Zero voltage axis while the remainder would drop below. Each curve of 8 represents Ithe integral of Equation 1 for some particular value of 0. The value of the integr-al at any instant of time can be ascertained from the Fresnel tables.

In Fig. 9 the dotted curve G is indicative of the pulse which occurs at point 5 coincidentally withv each zero beat; which pulse is the resul-t of squaring and adding together two quadraturerelated simultaneous integrated voltages per Fig. 8. Also included in Fig. 9 is a curve H which is the envelope, on the negative side, embracing the curves of Fig. 8 on the negative side of the zeroV voltage axis. It is tofbe understood that the complete envelope includes an additional curve, co m, plement-ary to curve H and disposed on the posi-V tive side of Ithe Zero voltage axis. The latterhas been omitted. from Fig.9 because it would serve no useful purpose .to show it.

It will nowbe shown mathematically that curve G is independent of the magnitude of the phase angle which may happen to obtain between .the

component waves which com-bine to form the (uitli-` puts of the respective second-mixers, and also that curve G is the square of envelope H. From the mathematical demonstration which follows it will be seen that the pulses appearing 4at point 5 are wholly independent of the magnitude of said phase angle, and that said pulses are, according-v ly, all alike.

y Given the function f1 (ar) :I cos (x2-+0) da' (1.1)

A second form of the function differing in phase from (11) by 90 is:

f2 (x) J' cos :c2-|-0+% dx f sin (x-I-B) dat It is desired to show that =cos 0 J' cos :tcZx-sin 0 f sin z'fdzv Simplifying, let

C= f cos :v2dx and let f S f sin zzdx Then l f1(:v)=C cos -S sin 0 (14) Similarly,

f2(x) =C sin @+S cos 0 (15) Hence,

[f1(:c)]2+[f2(x)]2= (C' cos 0-S sin (D2-I- (C' sin @+S cos 0)2=C'2+S2=f3(a:) (16) fand f3 (zu) is obviously independent of 0.

. and 56.

y always of 13 AIt will now be shown `tliat curve G is the `sqrutre of the envelope H-whi'c'h'is further proof that the pulse G isindependent of the -p`hasearlgled I Let .(15) represent the general function. 'To determine the amplitude of the envelopevat any point fr; the function must be maximized with re- S 'COS of tube .20 is-connected through capaci-tor 53 with the-control :grid of tube 54. The control grids of both-detector tubes: 52 and 5`ll'are biased to cut-oth` When the grid of phase inverter tube 2% goes vIlegative it-causes the control grid of tube u52 to go positive. while the grid of tube 5d remains below cutoff. When on the other hand, the `grid of tube .goes positive thecontrol grid of tube 52 remains both tube sections lnunc:inductive 1,4? described in the co -pending application of David? E.5Sunsteill, Serial Number $534,384, -ledMay 5",-` 1=944,..now"PatentvNumber 2,436,235.-

The phase inverter 2l and blo king; oscillator 2t serve to produce morepowerful .pulses than are readilyobtainable at point 5. While saidpulse inverter and :blocking oscillator are 'important as respects the entire calibrating system including the particular recording mechanism actually employed, fit will 'be .evident that they'constitu-te more or lessioptional equipment insofar as my fundamental invention is concerned. The function of phase inverter V2l is "to convert `the negative pulses appearing atipoint 5 .into positive pulses for ap plication lto grid :'59 of the blocking oscillator tube; The mode ofoperationof the blocking `oscillator is as follows:

The-.two cathodes are normally maintained? at a potential. which is sufficiently negativ-e 'to render .A` positive pulse Aimpressed lon grid 59 institutes a current ow through :primary winding Bil of transformer i 6l. This inducesin secondary winding `6.2 a voltage which renders grid 53 positive and thusgives rise to aplate current flow in the v'rightshand sectionot the.tubeA Thus, the right-hand section"` of the bloc-king oscillator `tube operates regenerai tively, :by virtue of feed-back through transformer below cut-orfA while the control grid of tube 54 goes positive. As La result, detector tubes 52 and 54 respond respectively to negative and positive voltagesion the grid of tube 20 and consequently, the voltage produced at junction 55 v-ia coupling condenser 5l` is the lsecond harmonic `or square of the integrated voltage impressed upon the grid of tube 20--but increased to theextent of whatever ,gain maybe afforded by the detector tubesl The operation of phase inverter 2:3 andsquarelaw .stage Z4 isexactly the same as that of inverter Eiland square-law stage 2|..

The voltage at junction 55 is equal to but 90 out of. phase withv the voltage at junction 55,y and these two quadrature-.related voltages are, as will he apparent, always negative. They are impressed uponopposite resistor terminals of potentiometer 25 and, consequently, the voltage at point 5 is always the mean between the two voltages at 55 The contactor oipotentiometer 25 is made` adjustable to compensate for any possible inequality between the outputsof the two squarelaw stages. The recurrent pulses at point A5 are the same magnitude and ofthe sameY polarity, and they coincide, to a very` `high degree of accuracy with `the zero beat, occurrences. Any recording mechanism may be employed which is susceptible of being triggered directly or indirectly by' thepulses occurring at point '5 and capable of recording coincident instantaneous dial read-` ings* of the frequency meter under calibration. For `a 'description ofv the recording mechanism actually yenfiployed seegthe aforementioned fEl'ectropics` article. Anotherfsuitable recorder is 6l, and, as a result, 'the .potential atfjunctiond and output terminal. l rises toV a high positive value :det-erminedby the plate .saturation ofthe tube. When tube saturation occurs 'thechange of current through primary VBil cease's and grid yl-Bis quickly .restored to its Vnormal'potential---whic"l"i completes the operating `cycle of the oscillato'r- To achieve the required dial scale precision mentioned lat :the beginning .of thisspecification itisnecessary to maire `certain that triggeringiof the recorder be effected` within a `tolerance lof 14120 cycles when lthe output of 'the `meter"und-er.

calibration :is at 'the low fend of the 121.0-4-D'mc. band, and within .a tolerance of i4() cycles -atthe high endifof said'bande-the permissible deviation being proportionate for allintermediate points; and the same degree ofdial sca-le precision must be maintained with respect to the lowband. Havirig those facts in mind, it is now lin order fto ex# plain the purpose and effectof tripling or otherwise multiplying the output frequency oi v4I.

amplersr yand I3. 'The objective is to increase i the steepness of wave-frontof the pulses at `point 5, as represented by curve G, Fig. 9, `'so that a dependable triggering voltage will `occur suiciently close to` each Izero beat to ensure that each" di`a1 reading is accurate Within the prescribed limits.`

Zin Fig-s. `8 and!) thezero voltage axesaredivided into units-of time, -and fthe interval t `laid oft' thereon corresponds toa period oi" .0036 sec. either side of zero beat or a total of .0072 sec. The latter corresponds to a 4"() cycle frequency change in the high band output of the frequency meter' or a 20 cycle changent eitherside of zero beat-which, asfsaidbefore, is the maximum allowable deviation at the low endof the high band if accu-racyof calibration is to be vmaintained within the permittedtolerancel 'I'f curveG is sufficiently steep wit-'hin the interval f't, it follows that the changeof i voltagefa't point 5, within the allowed time,` will be vsuilicient to ensure that triggering of the` recorder will always occur closely enoughto exact zero beatto meet the prescribed requirements. Y From inspection of 4Equation l it will be seen than the time required for the 'beat togo through that distinctive periodadjacent `zero beat during which "theintegrated 'beat voltage (Figbl v arately squaring said last rises or falls sharply is proportional t /-fwhich is the frate of change of frequency of the frequency meter under calibration. Hence, by increasing the value of R through frequency multiplication, the slope of curve Gis increased. `It niight erroneously be imagined that the frequency triplers serve merely to offset, in part, a loss due to the frequency step-down effected bynrst-mixers G and i2, but such is not the case. This is explained bythe fact that the frequency rate of change of the I. F'. output of first-mixer 1 is the same as the rate of change of the high band output of the frequency meter under calibration. Hence, no loss of calibration precision accrues from the frequencyreduction which precedes the frequency multiplication If the first-mixers and I. F. amplifiers were eliminated, thus sacrificing the innate advantages of the superheterodyne, it would still be necessary to introduce the same amount of frefi quency multiplication in order to realize the same steepness of wave front (curve G, Fig. 9) asis accomplished with the arrangement illustrated. f A i `It will be observed that localgoscillator it is provided with automatic frequency controlV connected to I. F. amplifier I3. The object in so doing 'is to ensureconstancy of the 1440 k, c. output 4of frequency tripler M so that transformer 42, which is tuned to 1440 k.v c., will produce a constant 90 phase shift. It is ofvinterest to note that in the absence of the first heterodyning operation the phase shifting transformer 42 would have to be made tunable throughout the 2.04.0 `megacycle band@ l.

-I claim: 'y il; The method of indicating frequency coincidence which' comprises combining two signal waves to produce a first beat waye which passes through `zero frequency, simultaneously combining said signal waves to produce a second beatv wave which is approximately ninety-de' grees phase-displaced from said first beat wave andwhich passes through zero frequency coi-nstantaneously with said first beat wave, utilizing said beat waves, separately and concurrently, each to produce, individually, a wave characterized by a pronounced `amplitude change marking the occurrence of zero'frequency, sepmentioned waves, concurrently, and adding the squared `waves to` produce avoltage pulse. l,

v2V. The method of indicating frequency coincidence which comprisesvcombining two signal Waves to produce a first beat wave which passes through zero frequency, simultaneously combining said signal waves to produce a second beat Wave which is approximately ninety degrees phase-displaced from Said Vfirst beat wave and` which passes. through zero frequency coinstantaneously with said first beat wavesepa'rately integrating said beat waves, and combining thel integrated beat waves to produce a voltage pulse.

3. The method of indicating frequency coincidence'which comprises combiningv two signal waves to produce a first beat wave which passes through zero frequency, simultaneously combining said signal waves to produce a second beat wave which is approximately ninetydegrees phase-displaced from said iirst beatl wave and which passes through zero frequency coinstantaneously with said first beat wave, utilizing said beatk waves, separately and concurrently, each to produce, individually, a wave characterized by a pronounced amplitude change marking the f 1615 occurrence of zero frequency, andcombining `the last-mentioned waves to produce aA voltage pulse.

4. The method of indicating frequencycoincidence as between two signal waves, when thev frequency of at least one of said waves is conf` tinuously changing, which method comprises,

as a rst step, combining the two signal waves` to` produce a first beat wave which passes through zero frequency, and simultaneously combining said signal waves to produce a second beat wave which is approximately ninety degrees phase-displaced from said first beat wave and which passes through zero frequency coinstantaneously with said first beat wave; as a second step, utilizing said beat waves, separately and concurrently, each to produce, individually, a wave characterized by a pronounced amplitude change marking the occurrence of zero frequency; as a third step, separately squaring said last-mentioned waves concurrently; and, as a fourth step, adding the squared waves `to produce a voltage pulse. v

5. The method of indicating frequency coin` cidence as between two signal waves, when the r second step, utilizing said beat waves, separate-v 1y and concurrently, each to produce, indie' vidually, a wave characterized by apronounced amplitude change marking the occurrence of zero frequency; as a third step, impressing each off the last-mentioned waves, separately, upon networks whose outputs differ from propor` tionality with the inputs supplied thereto; and, as a fourth step, combining the outputsof said networks to produce a voltage pulse.

'6. The method of indicating frequency rcoincidence as between two signal waves, where the frequency of at least one' of said waves is con"- tinuously changing, which method comprises, Aas a rst step, combining the two signal-waves to produce a iirst vbeat wave which `passes through` zero frequency, and simultaneously combining said ond beat wave which Vis approximately ninety degrees phase-displaced from said firstv beatY wave and which passes through zero frequency coinstantaneously with said first beat wave 'as' a second step, simultaneously and separately in, tegrating said first and second `beatwaves; as a third step, simultaneously and separately squarfV ing the two integral waves; and, as a fourthv step, adding'the two squared integral Wavest'o produce a voltage pulse. "7. The method of indicating frequency coincidence as between two signal waves, where the fre-` quency of at least one of said waves is`continu-` ously changing, which method fcomprises.as a' first step, combining the two signal Waves to produce a first beat wave which passes through zero frequency, and. simultaneously combining, said signal which is approximately *ninetyl degrees phase-` displaced from said first beatwave and'wh'ich passes through zero frequency coinstantaneously with said first beat Wave; as a second step, simul-f taneously and separately integrating vsaid first signal waves to produce ya secand second beat waves; as a third step, impressing each of the two-integrated waves, separately,

arcanes continuously increasing or decreasing the output frequency of the frequency meter through a preldetermined frequency band while atthe same time varying the output frequency offa local i oscillator so as to maintain `a constant frequency difference between the frequency meter output and the local oscillator output-combining the output of the frequency meterwith that of the is Y impressing `uponthe last-mentioned mixer a second pair of signal waves which is `substantiallir identical with Athe first-mentioned `pair except i that they phase angle between the members `of the second vpair differs` by approximately ninety degrees `from the phase anglebetween the members of the iirstjpair, devices individual to said `mixers and connected, respectively, to the outputs thereof and operative rtoin'anslate `frequency variations into amplitude variations, a pair `of networks whose outputs are disproportionate totheir inputs, said networks having their inputsconnected, individuallyyto said devices, `andrecordlocal oscillator to produce a beat wave of ccni stant frequency, combining the output of the local oscillator, consecutively and individually, with each of a series of harmonic frequency waves to produce a recurrent series of alternating current beat signals, each havinga fcenterrfrequency equal to that of `said beat wave consecutively mixing a portion of each of said bea-t signals with a portion of said constant frequency beat wave to form a recurrent seriesfof first` lower frequency beat signals each of l which-` passes through zero frequency, phase shifting through an angle-of ninety degrees a portion ofsaidtconstant frequency beat wave, `consecutively mixing the phase-shifted portion of saidbeat wave with another portiont of each of said recurrentlseries of beat signals to form a `secondrecurrentseries of lower frequency beat signals veach of `whirzhlis in phase quadrature to the coincidentlfirstlower frequency beat signal and passes` through `zero frequency coinstantaneously with said `coincident first lower frequency beat signal, utilizingxsaid lower frequency beat signals, separately andconcurrently, to produce, individually, a wavegcharacterized by au pronounced amplitude change marking the occurrence `of zero frequency, separately squaring the last-mentioned waves, yand adding the squared waves to produce a Voltage pulse. ,l c

9. The method according to claim 4 including the additional steps of multiplying the `frequency of the constant frequency beat wave and of equally multiplying the `frequencytof the first-mentioned beat signals, said step being performed prior, in each case, to the recited mixing operations. t t, l y

10. The method according to claim llwlrerein the waves characterized bya pronounced ampli-` tude change marking the occurrence of `zerofrequency each are produced by integrating, respectivelyv and separately, the coincident lowerf fre- 4quency beat signals, prior, in, each` casa4 to the squaring operation. i

n. The method accor-dinero claim- 4 including" the `additional steps of multiplying the frequency of the constant frequency beatwave andiof equally multiplying the frequency of the first-.men-` tioned beat signals, said steps being performed prior, in each case, to the recited mixing operations,` said method beingy further distinguished `in that the quadrature-related` coincident lower `frequency beat signals are separately integrated and thereafter separately squared and then; added.

l2. In an automatic calibratingv-l` system; `a mixer, means for impressing upon said mixer a pair of signal waves to produce arstalternatf ing current beat signal whose frequency decreases from amaxi-mum value to kzero andlthenincreases to a maximum? value, another mixerrfmeans .for

` from a maximum ValuetoV Zero and then in`` ing means responsive to the conjoint outputs of said networks.

13. In 1an automatic calibratingsystem,` a mixer,

`means `for impressing upon `said mixer a pair of signal waves to produce a first alternating current beatlsignalwhose frequency `decreases from a maximum value to zero and then increases to a maximum Valueanother`mixer, means for impressing upon the last-mentioned `mixer a second pair of signal waves which is substantially identicalwith the `first-mentioned pair except that the phase angle between the members of `the second` pair differs by approximately ninety degrees from the phase angle between the members of therst pair,` said last-mentioned mixer being operative to produce a second ,alternating current beat signal substantially identical with the first-mentioned beat signal but in phase quadrature with respect thereto, a pair of integrators, each connected, individually, to the outputs `of said mixers a pair of networks whose outputs are disproportionate to their inputs, said" networks `having their inputs connected, individually, to said integrators, and recording means responsive to the conjoint outputsof said networks.

14:. In an automatic calibrating system, a mixer, means for impressing upon said mixer a pair of signal wavesto produce a rst alternating current beat signal whose frequency decreases creases to a maximum value,` another mixer, means for; impressing upon the last-mentioned mixer` asecond pair of signal waves which is substantially identical with the first-mentioned pair except that the `phase angle between the members of the second `pairdiffers by approx-` imately ninety degrees from the phase angle between the members of the first pair, said lastt n'lentionedr mixer being operative to produce a. second alternating current beat signal substantially identica-lwth the first-mentioned beat signal but in phase quadrature with respect thereto, apairof integrators, each connected, individually, to the outputs of said mixers, a pair of square law detectors each having its input connectedthrough one of said integrators` to the output of`one of saidmixers, and recording means responsiveftc the conjoint outputs of said detectors.

,1.5. In "an automatic calibrating system, a mixer, means for impressing upon said mixer a `pair of signal waves to producea first alternat-` ing current beat `signal whose` frequency decreases from a maximum value to Zero and then increases to a` maximum value, another mixer,`

means for impressing upon the last-mentioned a `second pair of signal waves which is subs'tantially identical with the rst-mentionedpair except that the phase angle between the members of thesecondpair differs by approximately ninety degrees tfromthe phase anglebetween the members of the `iirstpain said last-mentioned mixer trasgressore@ produce a sewnd alternating current beat signal substantially identical with the nrst-mentioned beat signal but in phase quadrature with respect thereto, a pair of square law detectors having their inputs coupled, respecn tively and individually, to the outputs of said mixers, means individual to said mixers andinterposed, each, between one of said mixers and its associated square law detector for converting frequency changes of the respective beat signals into coincident voltage amplitude changes, and

' means for adding the outputs of said square law detectors.

l 16.'In an automatic calibrating system, a mixer, means for impressing upon said mixer a pair of signal waves to produce a rst alternating current beat signal whose frequency decreases from a maximum value to zero and then increases law detectors having their inputs coupled, respec-` tively and ind-ividually, to the outputs of said mixers, an integrator interposed between one of said mixers and its associated square law detector, a second integrator interposed between the other of ,said mixers and its associated square law detector, and means for adding the outputs of said square law detectors.

1'7. In a system for indicating frequency coincidence, a mixer, means for impressing upon said mixer, concurrently, a first signal wave and a Ysecond signal wave, at least one of which varies in frequency through a range including a center frequency which equals the coincident frequency of the other signal wave, a phase shifter for deriving from said second signal wave a third signal wave which is approximately ninety degrees phase-displaced from said second signal wave, another mixer, means for impressing upon the last-mentioned mixer, concurrently, said rst and third signal waves, two square law detectors, each having their inputs coupled individually to the outputs of the respective mixers, two integrators, each interposed, individually, between one of said mixers and its associated square `law detector, and a voltage adder interconnecting an output terminal of one of said detectors with a corresponding output terminal of the other detector. Y

18. In a system for indicating frequency coincidence, a first channel and a second channel, a signal generator operative to propagate simultaneously a plurality of'` signals of different predetermined frequencies, said iirst channel including a first-mixer and selector means for selecting said signals individually and impressing the same upon said first-mixer, said rst channelY also including an intermediate frequency ampliner following said first-mixer, said second chan-l nel including another first-mixer followed by an intermediate frequency amplifier, means for ,impressing upon the last-mentioned first-mixer a signal wave of continuously varying frequency,

the interposition' of said `phase shifter, another second-mixer coupled directly to lone of said Vbetween the members of channels and coupled to the other of said channels through said phase shifter, said second mixers being operative to combine signals from both said channels to produ-ce simultaneously two separate beat wave signals whichV are' substantiallyidentical except that they are mutually phase-displaced by approximately ninety degrees,

` v` each of said beat wave signals being characterized in that its frequency decreases from a maxi mum starting value through zero and then increases to a maximum final value, separate means responsive to the beat wave output of eachv sait second-mixers for producing a'pronounced voltage pulse coincidently with and marking the occurrence of zero frequency, a pair of square k law detectors for squaring said voltage pulses individually, and means for combining the out-'j' Vputs of said square law detectors.

19. In a system for indicating frequency coincidence, a rst channel and a second channel, a signal generator operative to propagate sirnu1` taneously a plurality of signals of different predetermined frequencies, said rstchannel including a first-mixer and selector means for selecting' said signals and impressing the same upon said first-mixer, said first channel also including an intermediate frequency amplier following said rstv mixer, said second channel including another rst-mixer followed by an intermediate frequency amplifier, means for impressing upon the last-mentioned first-mixer a signal wave of continuously varying frequency, a' local oscillator coupled to both first-mixers, a ninety degree phase shifter, a second-mixer coupled directly to both said channels without the interposition of said phase shifter, another second-'mixer coupled directly to one of said channels and coupled to the otherofsaid channels through said phase shifter, said second-mixers being operative to combine signals from both said channels to produce simultaneously two separate beat wave sig' nalswhich are substantially identical except that they are mutually phase-displaced by approximately ninety degrees, each of said beat wave signals being characterized in that its frequency decreases from a maximum starting value through zero and then increases to amaximum final value, separate means responsive to the beat wave output of each 'said second-mixers for 'producing, each, a pronounced amplitude change coincidentally with and marking the occurrence,

of zero frequency,A and means for utilizing the outputs of said separate means, conjointly, for indicating coincidence between said signal wave of continuously varying frequency and one of said signals of different predetermined frequencies.

" 20. In a system forindicating frequency coin-` cidence, a` rst channel and a second channel, a signal generator operative to propagate simultaneously a plurality of signals of different predeter-mined frequencies, said first channel ina local oscillator coupled to both first-mixers, a

cluding a first-mixer and selector"means for selecting said signals and impressing the same upon said first-mixer, said first channel also in cluding an intermediate frequency amplifier fole lowing said first-mixer, said second channel including another first-mixer followed by an intermediate frequency amplifier, means impressing upon the last-mentioned first-mixer a signal wave of continuously varying frequency, a local -oscillator coupled to both first-mixers, a

ninety degree phase shifter, a. second-mixer coul forV aasaocc pled directly/ttoy both `said channels without-the interposition. of said' phase "shifter, another second-mixer Vcoupled -directly Yto said rst channel and' coupled to `said :second channel through said phase shifter, said second-mixers.being` operative to combine signals from both said? channels to produce simultaneously two separate beat wave signals which are substantially .identical except that they vare `mutually phase-displaced '.by: ap proximately ninety degrees, each `of `said beat wave-signals being vcharacterized in `that its frequency decreases from a maximum starting value through zero. and thenincreases to a maximum final va1ue,nseparate.meansre'spon'sivet'o the beat wave output.I of Aeach said .second-mixers `for pro? dncirig, each, a pronounced amplitude :change coincidentally with andgmarking. the occurrence of zero.- frequency, and means for utilizing the outputs of said separate means, conjointly', for indicating coincidence between said signal wave of continuously varying frequency and one of said signals of different predetermined frequenciesi 21. In a system for indicating frequency coincidence, a firstL channel and a second channel, a signal generator operative to propagate simultaneously a plurality of signals of different predetermined frequencies, said iirst channel inciudin'g a Erst-mixer and an .amplifier whichis tunable 13o-select said signalslzand to impress'the 221 increasing or decreasing frequency, a local oscillator `coupled to fboth first-mixers, a ninety de grec phase shifter, a second-mixer coupled' directly `to both said channels without the interposition of said phase shifter, another secsame upon said first-mixer; said first channel also including `an intermediate frequency amplifier following said first-mixer, said second channel including another first-mixer followed `by another intermediate frequency amplifier,

means for impressing upon the last-mentioned first-mixer a signal Wave of continuously increasing or decreasing frequency, a local oscillator coupled to both first-mixers, a ninety degree phase shifter, a second-mixer coupled directly to both said channels without the interposition of said phase-shifter, another second-mixer coupled directly to one of said channels and coupled to the other of said channels through said phase shifter, said second-mixers being operative to combine signals from both said channels to produce simultaneously two separate beat Wave sgnals which are substantially identical except that they are mutually phase-displaced by approximately ninety degrees, each of said beat wave r.

signals being characterized in that its frequency decreases from a maximum starting value through zero and then increases to a maximum nal Value, a pair of square law detectors, each coupled, individually, to the output of one of said `second-mixers, a pair of integrators, each interposed between one of said second-mixers and its associated square law detector, and an adder for combining the outputs of said square law detectors.

22. In a system for indicating frequency coincidence, a first channel and a second channel, a signal generator operative to propagate, simultaneously, a plurality of signals of different predetermined frequencies, said first channel including a first-mixer and an amplifier which is tunable to select said signals and to impress the same upon said first-mixer, said rst channel also including an intermediate frequency amplider following said first-mixer, and a frequency multiplier, said second channel including another first-mixer followed by another intermediate frequency amplifier and another frequency multiplier, means for impressing upon thelast mentioned first-mixer a signal wave of continuously ondemixer' coupled' directly to one of said channels Aand coupled tothe other of said channels through said phase shifter, said second-mixers being :operative to combine signals from both said' 0 channels to produce simultaneously two separate beat Waver signals which are substantially identical except that they are mutually phase-displaced byapproximately ninety degrees, each of said beat wave signals being characterized in that its frequency decreases from a maximum starting Valuethrough zero'and then increases to a maxi-mum final valuea pair of square law detectors, each coupled, individually, to the output of one of said' second-mixers, a pair of integrators, each interposed between one of said second- `mixers and its 'associated square law detector, and an add-error combining the outputs of said square 'lawvv detectors.

` '23. In a system for indicating frequency coino cidence,` a rst channel and a second channel, said first channel including a first-mixer, means for impressing, consecutively, upon said rst mixer `a series of signal waves of different predetermined frequencies, a local source of oscillations connected to said first-mixer, said source havingL an youtput frequency which is variable through a `predetermined frequency band, said` source and said means being so intcrcoupled that `as the frequency of said source progresses from end to end of said band it beats, successively, with each one of said signal waves while the latter is being impressed upon said first-mixer by said means, the output of said first-mixer being a series of beat frequency signals, said second channel including another first-mixer, means for impressing upon the last-mentioned irst-mixer a sign al wave of continuously increasing or decreasing frequency, said last-mentioned first-mixer being also connected to said source` of ocillations,

l a ninety degree phase shifter. a seco-ndemixer di-` rectly connected to both said channels without the interpositon of said phase shifter, another second-mixer directly connected to one of said channels and connected to the other of said channels through said phase shifter, the output of each of i said second-mixers being a series of discrete beat Wave signals each characterized in that its frequency decreases from a maximum starting value through zero and then increases to a maximum discrete beat Wave signals each being in phase quadrature to the concurrent member of the other said series, a pair of square law detectors, each connected individually to the output of one of said second-mixers, means individual to and interposed between each of said` second-mixers and its associated square law detector for translating the change of frequency in the outputs of the secondmixers, at and immediately adiacent each zero beat, into pronounced amplitude changes each of which marks the occurrence of a zero beat, and means for adding the outputs of said square law detectors.

24. In a system for indicating frequency coincidence, a first channel and a second channel, said first channel including a first-mixer followed by an intermediate frequency amplifier together with a frequency multiplier and an amplitude limiter, means for impressing, consecutively, upon said first-mixer a series of high frequency signal final valuethe members of each said series of Waves of different predetermined frequencies, a local oscillator connected to said first-mixer, said local oscillator having an output frequency which Varies continuously through a predetermined frequency band, said oscillator and said means being so intercoupled that as the frequency of said oscillator progresses from end to end of its band its output beats, successively, With each of said sig nal Waves while the latter is being impressed upon said rst-mixer by said means, the output of said first-mixer being a series of intermediate frequency beat signals, said second channel including another first-mixer followed by another intermediate frequency amplifier together with another frequency multiplier and another amplitude limiter, means for impressing upon the lastmentioned first-mixer a signal Wave of continuously increasing or decreasing frequency, said last-mentioned first-mixer being also connected to said local oscillator, said local oscillator being gang-connected to both said means so as to maintain a constant intermediate frequency in said second channel and a predetermined range of frequency ydilerences `between the local oscillator output and each successive one of said signal waves during the time the latter is being impressed upon the rst-mentioned first-mixer, a ninety degree phase shifter, a second-mixer directly connected to the outputs of both said channels Without the interposition of said phase shifter, another second-mixer directly connected to the output of one of said channels and connected tothe output of the other of saidchannels through said phase shifter,I the output of Y each of said second-mixers being a series of discrete beat Wave signals, each characterized in that its frequency decreases from a maximum starting value through zero and then increases to a maximum nal value, the said signals of one said series each being in phase quadrature to the concurrent signal of the other said series, a pair of square law detectors, each connected individually to the output of one of said second-mixers, means individually to and interposed between each of said second-mixers and its associated square law detector for translating the change of frequency in the outputs of the second-mixers, at andimmediately adjacent each zero beat, into pronounced amplitude changes each of which marks the occurrence of a zero beat, and means for adding the outputs of said square law detectors.r

- f DAVID B. SMITH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,931,873 Marrison Oct.' 24, 1933 1,934,400 Bollman Nov. 7, 1933 2,044,749 Usselman June 16, 1936 2,086,892 Barton July 13, 1937 2,207,540 Hansell July 9, 1940 

